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I am finding the fuel consumption on my Mitsubishi Airtrek varies considerably when the air conditioning is used in mainly City driving. I drive approx 30 kilometres per day.

The consumption increases from around 12 litres per 100km's to 16 litres per 100km's, a jump of over 30% which seems very excessive to me.

Is this to be expected or a sign of some issue with the Air conditioning unit?

The car is serviced regularly and I have been told there is nothing wrong with the car, and figures like that can be expected.

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From the "Ask Jack" archives - 14 December 2009

I am surprised to hear of the difference in fuel consumption with the air-conditioning in use. I have carried out my own tests on a I year old car and found insignificant differences to be honest (under 5%).

The more modern vehicles have much improved and efficient air-conditioning systems, all designed to do the job with reduced effect on the engine load. The air-conditioning compressors for example are far more efficient than they use to be. Remember any device that is connected to the engine via a belt such as air-conditioning, power steering and an alternator has a cumulative effect on fuel consumption.

A phone call or visit to a air-conditioning specialist may be an option to ensure the system is working efficiently and the compressor is not being overworked unnecessarily. These systems need a top up of air-con gas occasionally.